When red-hot isn’t enough: New government heat risk tool sets magenta as most dangerous level

Forget about red hot. A new color-coded heat warning system relies on magenta to alert Americans to the most dangerous conditions they may see this summer.

The National Weather Service and the Centers for Disease Control and Prevention on Monday — Earth Day — presented a new online heat risk system that combines meteorological and medical risk factors with a seven-day forecast that’s simplified and color-coded for a warming world of worsening heat waves.

“For the first time we’ll be able to know how hot is too hot for health and not just for today but for coming weeks,” Dr. Ari Bernstein, director of the National Center for Environmental Health, said at a joint news conference by government health and weather agencies.

Magenta is the worst and deadliest of five heat threat categories, hitting everybody with what the agencies are calling “rare and/or long-duration extreme heat with little to no overnight relief.” It’s a step higher than red, considered a major risk, which hurts anyone without adequate cooling and hydration and has impacts reverberating through the health care system and some industries. Red is used when a day falls within the top 5% hottest in a particular location for a particular date; when other factors come into play, the alert level may bump even higher to magenta, weather service officials said.

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On the other hand, pale green is little to no risk. Yellow is minor risk, mostly to the very young, old, sick and pregnant. Orange is moderate risk, mostly hurting people who are sensitive to heat, especially those without cooling, such as the homeless.

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The five categories rest on strict science-set numerical thresholds, like the Saffir Simpson hurricane scale that is familiar for its Category 1 through 5 terminology, though the heat version is specific to location, said National Weather Service Director Ken Graham. The thresholds were calculated using local weather data, local climatology that shows what people are used to at certain locations at each time of year, and localized heath and medical data for when heat illnesses and deaths show up in the area, he said.